Conventionally, a hydraulic drive system for a working machine has been designed to permit detecting clogging of a filter in an exhaust gas purification device. When the working machine is in a non-operated state at the time of detection of clogging, the hydraulic drive system is set to automatically conduct forced regeneration. This forced regeneration conducted by the hydraulic drive system increases the delivery flow rate and delivery pressure of a hydraulic pump to increase the engine output, whereby the temperature of exhaust gas is raised. This increase in engine output leads to a rise in the temperature of exhaust gas. As a consequence, particulate matter with which the filter is clogged is burnt with the heat of the exhaust gas and is removed from the filter (see Patent Document 1).